Vehicle tracking using an automatic vehicle location (AVL) system is well known in the art. In a typical AVL system, each of a plurality of vehicles communicates its position to a base station. The position of each of the vehicles is commonly superimposed over a simplified map to provide a graphical display of the position of the vehicles with respect to the underlying simplified map. An AVL system can also be used to dramatically enhance the function of a computer aided dispatch (CAD) system.
In one use of a CAD system, a system operator dispatches a courier vehicle to a reported location to pick up a passenger and to carry the passenger to a designated location such as a car rental agency office. For example, a operator using a CAD system receives a request that a passenger is to be picked up at a specific location. By accessing an AVL system, the CAD operator is able to visually determine which of the displayed vehicles is positioned near or nearest to the location of the caller. However, an AVL system cannot determine the exact position of the vehicle when the vehicle is in an area in which the vehicle is obstructed from receiving position locating signals. For example, when the vehicle is on a road, highway, bridge or parking lot that has multiple levels, an overlying level can prevent the vehicle from receiving position locating signals. Such as position locating signals include, for example, satellite based position locating signals of the Global Positioning System (GPS). Therefore, although an AVL system can enhance the functionality of a CAD system, a conventional AVL system can not always reliably determine which of a plurality of vehicles can respond most quickly to an event. The shortcomings of a combined AVL and CAD system are especially prevalent in an environment containing complex features such as airports. This limitation on AVL and CAD systems is particularly problematic for taxi services, car rental services, and other transportation services that operate extensively in airports. In an airport environment, for example, it is critical that the operator know whether the vehicle is at the level at which passengers depart (departure level) or whether the vehicle is at the level at which passengers arrive (arrival level). Other signal obstructing environments include, for example, overpasses, tunnels and bridges in large metropolitan areas such as San Francisco, New York, Los Angeles, Boston, and the like.
As mentioned above, environments that contain multi-level structures often include structures that block the reception of GPS signals. For example, when a vehicle is on any level other that the top level of a road, overpass or bridge, the upper levels block reception of GPS signals. In addition, in complex environments such as those which typically contain multi-level structures, even the top level of the structure usually contains obstructions such as bridge spans, cables, buildings, pylons and other similar structures which can block reception of GPS signals. Thus, when multi-level structures are encountered, prior art CAD systems or combined AVL and CAD systems often cannot determine the exact location of the vehicle even when the vehicle is on the top level of the structure. The CAD operator must decide which of a plurality of available vehicles can respond most quickly to an event. That is, the CAD operator must consider situations and conditions that will affect each vehicle during its travel route such as vehicle impeding barriers, traffic, geographic barriers, congestion and regularly occurring events when determining which vehicle to dispatch to the event. As a result of not knowing the precise location of all vehicles, CAD system operators may be forced to guess or estimate the exact location of a vehicle. To make matters even worse for CAD system or combined AVL and CAD system operators, dispatch decisions must be made under rigorous time constraints. Hence, even well trained CAD operators may mistakenly dispatch the wrong vehicle or a slower responding vehicle to a location.
Although an operator may be able to determine the level on which a vehicle is located by referencing a destination log of a particular vehicle, such a determination is simply an educated guess. Furthermore, due to time constraints, AVL and CAD system operators typically will not have the time to refer to the destination log prior to determining which vehicle to dispatch because of time constraints.
Furthermore, although signals from GPS position information can be used to determine the altitude of a particular vehicle, GPS systems often cannot calculate altitude with a high enough accuracy to determine whether a vehicle is on, for example, the first or second level of a multi-level structure. This is primarily due to the fact that vehicles typically receive from satellites located within a narrow range directly above the vehicle. It is difficult to receive satellite signals from satellites located near the horizon. Therefore, the error in the calculated altitude is high, particularly when structures limit the reception of satellite signals. In addition, the difference in height between different levels of multi-level structures is not great, typically ranging less than twenty feet. Thus, the difference in height is usually less than the error of the calculated altitude. Therefore, prior art GPS systems cannot reliably determine at which level of a multi level structure the vehicle is located, even when the vehicle is able to receive GPS signals.
In an attempt to solve problems associated with multi-level structures and nearly coincident roadways, recent prior art systems have used transponders, typically referred to as "signposts" located on each level of the multi-level structure. These signposts send a signal to the base station once the vehicle passes the signpost. The signpost indicates the level at which the vehicle is located. This solution is expensive since signposts must be installed at each level of each multi-level structure. For example, with respect to airports, a signpost must be installed at each departure level and at each arrival level of each terminal. Signposts also require power. Therefore, electrical cable, for example, must be run to each signpost. Signposts are unique to and separately owned by each AVL or CAD system. Thus, each different AVL and CAD system must install its own signposts that work with its vehicles and its particular AVL and CAD system. It has recently become much more difficult to obtain permission to install signposts. This is particularly true at airports due to the number of requests for new installations, and due to restrictions on radio frequency and mechanical clutter at airport locations. Permission to install a new signpost now takes months to obtain, and in some cases, permission is not granted at all.
Other methods which could be used to determine the path of a vehicle traveling through a multi-level structure include the use of altimeters and inclinometers. However, altimeters are expensive and they must be repeatedly re calibrated to reflect current atmospheric conditions. Inclinometers are difficult to use and they produce unreliable results.
Thus, a need exists for a system which swiftly and automatically determines the location of a vehicle when the vehicle is on a multi-level structure, a system which can function in conjunction with an existing AVL or CAD system, and a system which reduces CAD system operator decision making processes.